Antiresonant waveguide apparatus for periodically selecting a series of at least one optical wavelength from an incoming light signal

ABSTRACT

The antiresonant waveguide apparatus is for periodically selecting a first series of at least one optical wavelength from a first incoming light signal. It comprises a first waveguide having an input for receiving the incoming light signal, the first waveguide having guiding mirrors for guiding the incoming light signal, one of the guiding mirrors being a first partial reflectivity mirror; a second waveguide having guiding mirrors for guiding an outputting light signal containing the first series of at least one wavelength; and a first Fabry-Perot resonator adjacent to the first partial reflectivity mirror, and forming one of the guiding mirrors of the second waveguide. The Fabry-Perot resonator is a second partial reflectivity mirror for the second waveguide. The Fabry-Perot resonator has a predetermined thickness determining the first series of at least one optical wavelength transmitted through the Fabry-Perot resonator from the first waveguide to the second waveguide.

FIELD OF THE INVENTION

The present invention is concerned with an antiresonant waveguideapparatus for periodically selecting a series of at least one opticalwavelength from an incoming light signal. More specifically, the presentinvention can be used either as multiplexer/demultiplexer, an add/dropfilter, or a narrow band filter of optical wavelengths.

BACKGROUND OF THE INVENTION

Known in the art, there is the U.S. Pat. No. 5,343,542 of Jeffrey A.Kash et al, granted on Aug. 30, 1994 in which there is described atapered. Fabry-Perot waveguide optical demultiplexer apparatus. Theapparatus provides a waveguide optical demultiplexer or spectrometer forapplications in optical communications. The apparatus can select atleast one optical wavelength from a plurality of optical wavelengths.The two major elements of this apparatus are a waveguide having apartial mirror along its length to reflect optical wavelengths therein,and an optical resonator where one of its resonating mirrors is thepartial mirror of the waveguide. Selected wavelengths are then extractedfrom the waveguide and resonated in the resonator.

One drawback of the apparatus described in Kash is that, after theextraction of the selected wavelengths from the waveguide, the only wayto collect the selected wavelengths from the resonator for further useis to place a photodetecting array along the resonator. These selectedwavelengths can then be converted into electrical signals via thephotodetecting array.

Also known in the art, there is the article by W. P. Huang et al,published by the Journal of Lightwave Technology, Vol. 11, No. 7, July1993, in which there is described an ARROW-based optical wavelengthfilter. This filter comprises two asymmetrical waveguides wherein onlyone wavelength is selected.

One problem with the apparatus described in Huang is that it cannotselect several wavelengths simultaneously. It is also very difficult tocouple asymmetric waveguides over to optical fibres. Furthermore, thisapparatus requires three different materials, which is incompatible withstandard optical integration.

Also known in the art, there are the U.S. Pat. Nos. 3,666,351;4,196,396; 4,229,710; 4,239,329; 4,745,607; 5,212,584; 5,272,711;5,276,748; 5,287,214; 5,317,655; 5,367,582; 5,386,426; 5,402,509 whichdescribe different apparatuses relating to optical devices.

None of the above patents provide the necessary means for easilycoupling in a simple manner at least one selected optical wavelengthfrom a first waveguide over to a second waveguide so that such selectedoptical wavelengths can then be easily redirected into an optical fibre.

An object of the present invention is to provide an apparatus fordirectly coupling at least one selected optical wavelength from a firstwaveguide over to a second waveguide so that such selected opticalwavelengths can then be easily redirected into an optical fibre.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an antiresonantwaveguide apparatus for periodically selecting a first series of atleast one optical wavelength from a first incoming light signal,comprising:

a first waveguide having an input for receiving the incoming lightsignal, the first waveguide having guiding mirrors for guiding theincoming light signal, one of the guiding mirrors being a first partialreflectivity mirror;

a second waveguide having guiding mirrors for guiding an outputtinglight signal containing the first series of at least one wavelength; and

a first Fabry-Perot resonator adjacent to the first partial reflectivitymirror, and forming one of the guiding mirrors of the second waveguide,the Fabry-Perot resonator being a second partial reflectivity mirror forthe second waveguide, the Fabry-Perot resonator having a predeterminedthickness determining the first series of at least one opticalwavelength transmitted through the Fabry-Perot resonator from the firstwaveguide to the second waveguide.

According to the present invention, there is also provided a method forperiodically selecting a first series of at least one optical wavelengthfrom a first incoming light signal, comprising steps of:

receiving the incoming light signal by means of an input of a firstwaveguide, the first waveguide having guiding mirrors for guiding theincoming light signal, one of the guiding mirrors being a first partialreflectivity mirror;

guiding an outputting light signal containing the first series of atleast one wavelength by means of a second waveguide having guidingmirrors; and

providing a first Fabry-Perot resonator adjacent to the first partialreflectivity mirror, and forming one of the guiding mirrors of thesecond waveguide, the Fabry-Perot resonator being a second partialreflectivity mirror for the second waveguide, the Fabry-Perot resonatorhaving a predetermined thickness determining the first series of atleast one optical wavelength transmitted through the Fabry-Perotresonator from the first waveguide to the second waveguide.

The objects, advantages and other features of the present invention willbecome more apparent upon reading of the following non restrictivedescription of a preferred embodiment thereof given for the purpose ofexemplification only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of the tapered Fabry-Perot waveguideoptical demultiplexer apparatus known in prior art;

FIG. 2 is a schematic top view of the tapered Fabry-Perot waveguideoptical demultiplexer apparatus known in prior art;

FIG. 3 is a schematic front view of an apparatus according to thepresent invention;

FIG. 4 is a schematic top view of the apparatus shown in FIG. 3;

FIG. 5 is a diagram of the refractive indexes and thicknesses ofelements of the apparatus shown in FIGS. 3 and 4 with respect toposition x;

FIGS. 6, (a) and (b), are diagrams showing output signal intensitieswith respect to wavelength;

FIG. 7 is a schematic top view of an apparatus according to the presentinvention;

FIG. 8 is a diagram of the refractive indexes and thicknesses ofelements of an apparatus according to the present invention with respectto position x;

FIGS. 9, (a), (b) and (c), are diagrams showing output signalintensities with respect to wavelength; and

FIG. 10 is a perspective view of an apparatus according to the presentinvention; and

FIGS. 11a and 11b, in some instances referred to collectively as FIG.11, are diagrams showing output signal intensities with respect towavelength.

DETAILED DESCRIPTION OF THE DRAWINGS

Similar elements throughout the drawings are identified by the samereference numbers. Referring now to FIGS. 1 and 2, there is shown in aschematic manner a device of the prior art, similar to the one shown inthe U.S. Pat. No. 5,343,542. This device comprises a Fabry-Perot 2resonator having a resonating cavity which is perpendicular to the lightpropagation direction of the light guided through the core of thewaveguide 4. The device also comprises first, second and third highreflectivity mirrors 6, 8 and 10. The first, second and thirdreflectivity mirrors 6, 8 and 10 are fabricated as distributed mirrorsor DBR mirrors. In this device, there is only one waveguide 4 and aFabry-Perot resonator 2. It has to be noted that the Fabry-Perotresonator 2 is provided with two high reflectivity mirrors 8 and 10.

In this device, the wavelengths λ₁, λ₂, λ₃ and λ₄ are selected andtrapped in the cavity of the resonator 2. To recuperate the wavelengthsλ₁, λ₂, λ₃ and λ₄, a photodetecting array of detectors 14 is mounted onthe side of the Fabry-Perot resonator 2 for converting the wavelengthsignals into electrical signals. It can be seen that the wavelengths λ₁,λ₂, λ₃ and λ₄ cannot be easily rerouted into an optical fibre. With thisdevice, it is not possible to do an add/drop filter for an opticalcommunication system.

Referring now to FIGS. 3 and 4, there is shown, in a schematic manner,an antiresonant waveguide apparatus according to the present invention.The apparatus is for periodically selecting a first series of at leastone optical wavelength from a first incoming light signal. In thispreferred embodiment, the incoming light signal has wavelength λ₁ to λ₆.The at least one wavelength includes wavelengths λ₂, λ₄ and λ₆.

The apparatus comprises a first waveguide 15 having an input 16 forreceiving the incoming light signal. The first waveguide 15 has guidingmirrors 18 and 20 for guiding the incoming light signal. One of theguiding mirrors 18 and 20 is a first partial reflectivity mirror 20.

A second waveguide 17 is provided. It has guiding mirrors 22 and 24 forguiding an outputting light signal containing the first series ofwavelengths λ₂, λ₄, and λ₆. The guiding mirror 22 is a Fabry-Perotresonator 22 which is adjacent to the first partial reflectivity mirror20. The Fabry-Perot resonator 22 is a partial reflectivity mirror 22 forthe second waveguide 17. The Fabry-Perot resonator 22 has apredetermined thickness determining the first series of opticalwavelengths λ₂, λ₄, and λ₆ transmitted through the Fabry-Perot resonator22 from the first waveguide 15 to the second waveguide 17.

The device of the prior art shown in FIGS. 1 and 2 will be compared tothe apparatus shown in FIGS. 3 and 4. In the prior art device, there isonly one waveguide 4, the Fabry-Perot resonator 2 is only used as aresonator and on both sides of the Fabry-Perot resonator 2 there arehigh reflectivity mirrors 8 and 10 whereas, in the apparatus shown inFIGS. 3 and 4, there are two waveguides 15 and 17 that are coupled, theFabry-Perot resonator 22 is used as a reflectivity mirror for the secondwaveguide 17 and the Fabry-Perot resonator is adjacent to the lowreflectivity mirror 20.

In the apparatus according to the present invention and shown in FIGS. 3and 4, the Fabry-Perot resonator 22 is used to reflect the selectedwavelengths into the second waveguide 17 so that they can be easilyrerouted into an optical fibre (not shown) from the second waveguide 17whereas, in the prior art device, the selected wavelengths are trappedin the Fabry-Perot resonator 2 and to extract the signals carried bythese wavelengths photoelectric converters 14 are needed. Also, theprior art device cannot be used to make an add/drop filter, amulti/demultiplexer or an optical filter whereas it is possible with theapparatus according to the present invention.

Referring now to FIG. 5, there is shown in a schematic manner, how theapparatus illustrated in FIGS. 3 and 4 is made. FIG. 5 shows therefractive indexes and the thicknesses of the elements of the apparatusshown in FIGS. 3 and 4 with respect to position x. The first waveguide15 has a core A and two mirrors 18 and 20. The two mirrors 18 and 20 area high reflectivity mirror 18 and the partial reflectivity mirror 20.The first high reflectivity mirror 18 includes three cladding layers 30.The first partial reflectivity mirror 20 includes one cladding layer 32.

The second waveguide 17 has a core B and two mirrors 22 and 24. The twomirrors are a high reflectivity mirror 24 and the partial reflectivitymirror 22 which is the Fabry-Perot resonator 22. The high reflectivitymirror 24 includes three cladding layers 34. The cores A and B of thewaveguides 15 and 17 have a similar thickness D_(c0) and a similarrefractive index n_(c0) which is different from the one n_(c1) of thecladding layers 30, 32 and 34, and of the Fabry-Perot resonator 22. Thedistance between two of the cladding layers 30 or 34 that are adjacentis D_(c0) /2. The distance between the Fabry-Perot resonator 22 and theadjacent cladding layer 32 is D_(c0) /2.

The cladding layers 30, 32 and 34 of the high reflectivity mirrors 18and 24 and of the first partial reflectivity mirror 20 each has asimilar thickness D_(c11) determined by the following first equation:##EQU1## where λ_(c) is a communication bandwidth central wavelengthdetermined by operating condition chosen by a user, and N₁ is anantiresonance condition order determined by the operating condition. TheFabry-Perot resonator 22 has a thickness D_(c12) determined by thefollowing second equation: ##EQU2## where N₂ is calculated by means ofthe following third equation: ##EQU3## where λ_(d) is a wavelengthchosen by the user and to be transmitted through the Fabry-Perotresonator 22, and Δλ is the free spectral range of the device and it isdetermined by the operating condition. For example, if D_(c0) is 8 μm,n_(c0) is 1.52, n_(c1) is 1.57, Δλ is 11.94 nm, N₁ is 1, λ_(d) is 633 nmand λ_(c) is 633 nm then D_(c11) is 0.436 μm and D_(c12) is 42.1 μm.

The principle of operation will be explained by referring to theembodiment for the add/drop filter depicted in FIG. 5. The embodimentcan be considered as two coupled antiresonant reflecting opticalwaveguides or ARROW's, which are marked by the circles. The embodimentincorporates a Fabry-Perot resonator having a relatively thick layerwhere Fabry-Perot interference is such that only light at certainselected wavelengths couples efficiently from core A to core B. Light atother wavelengths goes on through to the output of core A.

As is well known from directional coupler theory, light from core A cancouple efficiently to core B only when the two are phase-matched oralmost phase-matched, i.e. when the real parts of the propagationconstants are nearly equal for the two waveguides taken separately. Bydesigning the cores to have substantially the same thickness D_(c0) =8μm, a good connection from the ARROW waveguide core to a single modefiber is guaranteed.

The wavelength selectivity of the apparatus is realized through thechoice of different cladding layer thicknesses for the two ARROW's, thisdifference in thickness being due to the thickness of the Fabry-Perotresonator. In order to obtain the best possible fabrication tolerancesfor the apparatus, the thicknesses of the cladding layers of thewaveguides have to be adjusted to the antiresonant condition. For theadjustment of the cladding layer thickness D_(c11) to the antiresonantcondition the approximation is given by the first equation.

The embodiment of an add/drop filter shown in FIG. 5 is based on apolymer material system where the core and cladding refractive indexesare 1.52 and 1.57 respectively. The effect of material dispersion andlosses were neglected in the present explanations. With the givenmaterial parameters one obtains the following cladding layer thicknessesD_(c11) =0.4 μm and D_(c12) =42.06 μm.

The reader should keep in mind that the first and second equations areapproximations and obtained thicknesses for the cladding layers are onlyestimates. Therefore the whole design for the structure was checked bycalculating the dispersion relation, i.e. the effective index or thereal part of the propagation constant as a function of the wavelengthfor ARROW's separately. From a well known transfer matrix method, fourequations for the determination of the propagation constant can beobtained. ##EQU4## where M_(ij) denote the matrix elements of thecomplete transfer matric for each single ARROW. It turns out that theestimates for the cladding layer thicknesses were quite good. Only acorrection for the thickness D_(c11) from 0.4 to 0.436 μm was necessaryto achieve phase-matching between the ARROW's.

With this correction the dispersion relation of the fundamental ARROWmode was calculated from the four equations. Over narrow bands centeredon 621, 633, 645 nm, etc . . . , efficient coupling takes place so thatpower will outcouple from core B after a certain coupling length. Atother wavelengths there is no phase-matching between ARROW's and theinjected light remains in core A.

Although we discuss only the case of TE-polarized light the whole designprocess, or the determination of the cladding layer thicknesses, so faris identical for the TE-polarized and the TM-polarized light. But thisis no longer valid for the determination of an apparatus operationlength or half beat length of an ARROW-based add/drop filter. For thispurpose we have to calculate the effective index of the symmetricn_(eff) ^(s) (λ) and antisymmetric supermode n_(eff) ^(a) (λ) for thecomplete structure. The supermodes are obtained also by means of thefour equations. The transfer matrix elements M_(ij) are now built up forthe complete structure. The difference in the effective indexes betweenthe symmetric and antisymmetric supermode gives the operation lengthL(λ) of the device as a function of the wavelength according to theformula: ##EQU5##

A central wavelength λ_(c) having a value of 633 nm was used todetermine the operation length. According to the polarization of theincoming light or TE/TM, we got half beat lengths of 50 mm/45 mm for theARROW add/drop filter.

This relatively long coupling length could not be reduced in the presentconfiguration without diminishing the spectral finesse. The couplinglength and the finesse are somehow intimately related. One way to reducethis length without affecting the finesse would be to reduce the coresize. But this would cause a mismatch in coupling to optical fibers.

Computations show a good tolerance to small changes in the antiresonantlayer thicknesses. This is a feature of an ARROW waveguide. Variationsof as much as 10% in the antiresonant layer thickness along the couplerwill not affect very much our selectivity except for a small loss in thefinesse. This is not the case for the thick layer which acts as aFabry-Perot resonator. Variations in its thickness will result in achange of the selected wavelength. To keep a good selectivity, thethickness of the Fabry-Perot resonator must remain under 0.1% ofvariation or lower than 0.4 μm. This is something really hard to achievein slab waveguides, but not impossible. A good lithographic setup couldprovide this kind of precision.

Referring now to FIG. 6, there is shown results of supermodecalculations for the embodiment shown in FIG. 5, where the outputintensity of core A and B are plotted as a function of input lightwavelength. One recognizes here the familiar Fabry-Perot periodicfiltering function. The peaks have a linewidth of 1.2 nm. The freespectral range was designed to be 12 nm by choosing D_(c12) to be 42.06μm. The finesse of the Fabry-Perot filtering is therefore 10. Thisfinesse would enable one to select one channel out of a uniform comb ofas many as five.

Note that potential uses of this filter are not restricted to uniformcombs of input wavelengths. By cascading embodiments as the one shown inFIG. 5 with different free spectral ranges one could take advantage of avernier effect and obtain much more widely separated transmission peaks.

The side lobe 21 suppression ratio in FIG. 6b falls in the range 8 to 16dBs. Higher suppression ratios could be obtained in systems applicationsby cascading two or more filters as the one shown in FIG. 5.

Referring now to FIG. 7, there is shown an apparatus similar to the oneshown in FIG. 4 except that the second waveguide 17 comprises an input40 for receiving a second incoming light signal including wavelengths λ₇and λ₈. If λ₇ and λ₈ are optical wavelengths of the first series of atleast one optical wavelengths, they will be coupled out of core 17 tocore 15 and so added to λ₁, λ₃, λ₅.

Referring now to FIG. 8, there is shown, in a schematic manner, howanother apparatus according to the present invention is made. FIG. 8shows the refractive indexes and thicknesses of the elements of theapparatus with respect to position x. The first waveguide 15 has asecond guiding mirror 19 that is a partial reflectivity mirror 19. Theapparatus further comprises a waveguide 42 having guiding mirrors 44 and46 for guiding a second outputting light signal containing a secondseries of at least one wavelength.

A second Fabry-Perot resonator 44 is provided. It is adjacent to thepartial reflectivity mirror 19, and forms one 44 of the guiding mirrors44 and 46 of the waveguide 42. The second Fabry-Perot resonator 44 is apartial reflectivity mirror 44 for the waveguide 42. The secondFabry-Perot resonator 44 has a predetermined thickness determining thesecond series of at least one optical wavelength transmitted through thesecond Fabry-Perot resonator 44 from the core A of the waveguide 15 tothe core C of the waveguide 42.

The embodiment shown in FIG. 5 fulfills the functionality of theconventional lumped element Fabry-Perot. We can take advantage of theintegrated optics to go beyond the capability of the embodiment shown inFIG. 5 and design a second embodiment of the ARROW add/drop filter suchas the one shown in FIG. 8 which drops two series of wavelengths atonce. In effect this dual filter can replace two embodiments such as theone shown in FIG. 5.

The embodiment of a dual ARROW add/drop filter is shown in FIG. 8.Basically, we have added a third ARROW to the embodiment of FIG. 5. As afirst approximation, the operation of the apparatus of the FIG. 8 can beunderstood as being simply the simultaneous operation of two ARROWfilters or two embodiment of the type shown in FIG. 5. The claddinglayer thickness D_(c13) of third ARROW 42 is chosen so that thewavelengths dropped into core C are 6 nm away from the wavelengthsdropped into core B. Using the order N=53 and the dropping wavelengthλ_(d) =627 nm we obtain as a good approximation for D_(c13) =41.66. Asbefore this thickness had to be corrected to D_(c13) =41.71 to achievephase-matching between the ARROW with core A and the ARROW with core Cat those wavelengths by means of the four equations.

From the field profiles of the symmetric and antisymmetric supermodes,it can be recognized that phase matched coupling between ARROW with coreA and ARROW with core B or C occurs independently for the droppedwavelengths λ_(d) ^(B) /λ_(d) ^(C). This can be summarized in thefollowing relations:

    n.sub.eff.sup.C (λ.sub.d.sup.B)≠n.sub.eff.sup.A (λ.sub.d.sup.B)=n.sub.eff.sup.B (λ.sub.d.sup.B)

    n.sub.eff.sup.C (λ.sub.d.sup.C)=n.sub.eff.sup.A (λ.sub.d.sup.C)≠n.sub.eff.sup.B (λ.sub.d.sup.C).

Referring now to FIG. 9, there are shown the output intensities as afunction of the wavelength for the embodiment of a dual ARROW add/dropfilter shown in FIG. 8. The results are the expected ones. The outputsignal characteristic of core B is identical to the one obtained withthe embodiment shown in FIG. 5. This underlines the fact that in theembodiment of an ARROW add/drop coupler as shown in FIG. 8, there is nointeraction between core A and core C. The output of core C ispractically identical to the output of core B, there is only a shift ofabout 6 nm.

Referring now to FIG. 10, there is shown, in a schematic manner, how anapparatus shown in FIG. 5 is made. FIG. 10 shows an optical wavelengthfilter based on asymmetric ARROW coupler in strip configuration. TheARROW filter of the type shown in FIG. 5 has been fabricated on anoxydized Si wafer. SiON of n=1.65 and 0.5 μm thickness was deposited onthis wafer by PECVD. After electron-beam direct writing the structurewas transferred to SiON by reactive ion etching or RIE. Typical stripwidths are in the submicrometer range, between 0.4 and 0.6 μm. Finallythe whole structure was covered over with SiO₂.

Referring now to FIG. 11, there are shown the output intensities as afunction of the wavelength for an embodiment of an ARROW filter that issimilar to the one shown in FIG. 10 except that the cores A and B havenot exactly the same thickness. Cores A and B can be viewed as beingslightly asymmetric. For example, cores A and B can have respectivelythicknesses of 15.0 μm and 16.5 μm. From the output signals, it can beappreciated that the finesse is increased.

The apparatus according to the present invention accomplishes aFabry-Perot filtering functionality by using the wavelength selectivecoupling between optical waveguides. By using a Fabry-Perot resonatorhaving a given thickness, selectivity is achieved despite the fact thatthe coupled waveguides have substantially equal core thicknesses, incontrast with filters based on conventional waveguides. For theapparatus according to the present invention, a resolution of 1.2 nm isobtained with a free spectral range of 12 nm. A dual output filter isalso described, it could drop or add two different wavelengths at oncein a wavelength division multiplexing system or a WDM system.

The apparatus according to the present invention is a planar waveguidestructure which utilizes Fabry-Perot resonators to achieve a fairly highand periodic wavelength selectivity, and which constitutes a first steptowards an integrated optic implementation of Fabry-Perot functionality.In contrast to the conventional Fabry-Perot, the multilayer waveguidestructure according to the present invention utilizes grazing incidencerays instead of normal incidence rays and provides two outputs insteadof one output.

With the present invention, it is possible to obtain a spectrally narrowselectivity such as 1 to 2 nm and a periodic selectivity such as 6 to 12nm that can be achieved in antiresonant reflecting optical waveguidestructures or ARROW structures by incorporating relatively thickFabry-Perot resonator or Fabry-Perot interference cladding layers. Twodevice configurations are proposed to achieve periodic selectivity byrelying on the wavelength selective coupling between two and three ARROWwaveguides which incorporate a Fabry-Perot resonator having thickFabry-Perot layers. Both structures can be used as add/drop filters. Theembodiment shown in FIG. 5 drops one series of wavelengths while theembodiment shown in FIG. 8 drops two series of wavelengths at once intodifferent output channels. The add function is accomplished by simplyreversing the direction of light in the input and output channels.

Although the invention has been described above in detail in theframework of a preferred embodiment, it should be understood that thescope of the present invention is to be determined by the appendedclaims.

We claim:
 1. An antiresonant waveguide apparatus for periodicallyselecting a first series of at least one optical wavelength from a firstincoming light signal, comprising:a first waveguide having an input forreceiving the incoming light signal, the first waveguide having guidingmirrors for guiding the incoming light signal, one of the guidingmirrors being a first partial reflectivity mirror; a second waveguidehaving guiding mirrors for guiding an outputting light signal containingthe first series of at least one wavelength; and a first Fabry-Perotresonator adjacent to the first partial reflectivity mirror, and formingone of the guiding mirrors of the second waveguide, the Fabry-Perotresonator being a second partial reflectivity mirror for the secondwaveguide, the Fabry-Perot resonator having a predetermined thicknessdetermining the first series of at least one optical wavelengthtransmitted through the Fabry-Perot resonator from the first waveguideto the second waveguide.
 2. An apparatus according to claim 1, whereinthe first waveguide having a second of its guiding mirrors that is athird partial reflectivity mirror, the apparatus further comprising:athird waveguide having guiding mirrors for guiding an outputting lightsignal containing a second series of at least one wavelength; and asecond Fabry-Perot resonator adjacent to the third partial reflectivitymirror, and forming one of the guiding mirrors of the third waveguide,the second Fabry-Perot resonator being a fourth partial reflectivitymirror for the third waveguide, the second Fabry-Perot resonator havinga predetermined thickness determining the second series of at least oneoptical wavelength transmitted through the second Fabry-Perot resonatorfrom the first waveguide to the third waveguide.
 3. An apparatusaccording to claim 1, wherein the second waveguide comprises an inputfor receiving a second incoming light signal, the second incoming lightsignal being within the first series of at least one optical wavelength,whereby the second incoming light signal is coupled out of the secondwaveguide to the first waveguide.
 4. An apparatus according to claim 1,wherein:the first waveguide has a core and two mirrors which are a firsthigh reflectivity mirror and the first partial reflectivity mirror, thefirst high reflectivity mirror including three cladding layers, thefirst partial reflectivity mirror including one cladding layer; thesecond waveguide has a core and two mirrors which are a second highreflectivity mirror and the second partial reflectivity mirror which isthe Fabry-Perot resonator, the second high reflectivity mirror includingthree cladding layers, the cores of the waveguides having a similarthickness D_(c0) and a similar refractive index n_(c0) which isdifferent from the one n_(c1) of the cladding layers of the highreflectivity mirrors and of the first partial reflectivity mirror; thedistance between two of the cladding layers that are adjacent is D_(c0)/2; the distance between the Fabry-Perot resonator and the adjacentcladding layer is D_(c0) /2; the cladding layers of the highreflectivity mirrors and of the first partial reflectivity mirror eachhas a similar thickness D_(c11) determined by the following firstequation: ##EQU6## where λ_(c) is a communication bandwidth centralwavelength determined by operating condition chosen by a user, and N₁ isan antiresonance condition order determined by the operating condition;the Fabry-Perot resonator has a thickness D_(c12) is determined by thefollowing second equation: ##EQU7## where N₂ is calculated by means ofthe following third equation: ##EQU8## where λ_(d) is a wavelengthchosen by the user and to be transmitted through the Fabry-Perotresonator, and Δλ is the free spectral range of the apparatus and it isdetermined by the operating condition.
 5. An apparatus according toclaim 4, wherein D_(c0) is 8 μm, n_(c0) is 1.52, n_(c1) is 1.57, Δλ is11.94 nm, N₁ is 1, λ_(d) is 633 nm and λ_(c) is 633 nm so that D_(c11)is 0.436 μm and D_(c12) is 42.1 μm.
 6. An apparatus according to claim1, wherein:the first waveguide has a core and two mirrors which are afirst high reflectivity mirror and the first partial reflectivitymirror, the first high reflectivity mirror including three claddinglayers, the first partial reflectivity mirror including one claddinglayer; the second waveguide has a core and two mirrors which are asecond high reflectivity mirror and the second partial reflectivitymirror which is the Fabry-Perot resonator, the second high reflectivitymirror including three cladding layers, the cores of the waveguideshaving thicknesses that are slightly asymmetric in that said thicknessesare slightly different.
 7. A method for periodically selecting a firstseries of at least one optical wavelength from a first incoming lightsignal, comprising steps of:receiving the incoming light signal by meansof an input of a first waveguide, the first waveguide having guidingmirrors for guiding the incoming light signal, one of the guidingmirrors being a first partial reflectivity mirror; guiding an outputtinglight signal containing the first series of at least one wavelength bymeans of a second waveguide having guiding mirrors; and providing afirst Fabry-Perot resonator adjacent to the first partial reflectivitymirror, and forming one of the guiding mirrors of the second waveguide,the Fabry-Perot resonator being a second partial reflectivity mirror forthe second waveguide, the Fabry-Perot resonator having a predeterminedthickness determining the first series of at least one opticalwavelength transmitted through the Fabry-Perot resonator from the firstwaveguide to the second waveguide.
 8. A method according to claim 7,wherein the first waveguide having a second of its guiding mirrors thatis a third partial reflectivity mirror, the method further comprisingsteps of:guiding an outputting light signal containing a second seriesof at least one wavelength by means of a third waveguide having guidingmirrors; and providing a second Fabry-Perot resonator adjacent to thethird partial reflectivity mirror, and forming one of the guidingmirrors of the third waveguide, the second Fabry-Perot resonator being afourth partial reflectivity mirror for the third waveguide, the secondFabry-Perot resonator having a predetermined thickness determining thesecond series of at least one optical wavelength transmitted through thesecond Fabry-Perot resonator from the first waveguide to the thirdwaveguide.
 9. A method according to claim 7, further comprising step ofreceiving a second incoming light signal by means of an input of thesecond waveguide, the second incoming light signal being within thefirst series of at least one optical wavelength, whereby the secondincoming light signal is coupled out of the second waveguide to thefirst waveguide.
 10. A method according to claim 7, wherein:the firstwaveguide has a core and two mirrors which are a first high reflectivitymirror and the first partial reflectivity mirror, the first highreflectivity mirror including three cladding layers, the first partialreflectivity mirror including one cladding layer; the second waveguidehas a core and two mirrors which are a second high reflectivity mirrorand the second partial reflectivity mirror which is the Fabry-Perotresonator, the second high reflectivity mirror including three claddinglayers, the cores of the waveguides having a similar thickness D_(c0)and a similar refractive index n_(c0) which is different from the onen_(c1) of the cladding layers of the high reflectivity mirrors and ofthe first partial reflectivity mirror; the distance between two of thecladding layers that are adjacent is D_(c0) /2; the distance between theFabry-Perot resonator and the adjacent cladding layer is D_(c0) /2; andthe cladding layers of the high reflectivity mirrors and of the firstpartial reflectivity mirror each has a similar thickness D_(c11) ;themethod further comprising steps of: determining the thickness D_(c11) bythe following first equation: ##EQU9## where λ_(c) is a communicationbandwidth central wavelength determined by operating condition chosen bya user, and N₁ is an antiresonance condition order determined by theoperating condition; and determining a thickness D_(c12) of theFabry-Perot resonator by the following second equation: ##EQU10## whereN₂ is calculated by means of the following third equation: ##EQU11##where λ_(d) is a wavelength chosen by the user and to be transmittedthrough the Fabry-Perot resonator, and Δλ is the free spectral range andis determined by the operating condition.